1. Field of the Invention
The present invention relates to a tracking servo system for use in an information recording disc playing apparatus. Apparatuses for playing an information recording disc, such as a video disc or a digital audio disc, must inevitably be provided with the tracking servo system for controlling the position of an information reading light spot of the pickup so that it accurately follows the recording track irrespective of the eccentricity of the disc, for example.
This tracking servo system constitutes the so-called closed loop system which generates a tracking error signal corresponding to the amount of deviation of the information reading light spot with respect to the recording track of the disc in a disc's radial direction, and performing position control relative to the recording track by driving an actuator for moving the information reading light spot in the radial direction in response to the tracking error signal. Furthermore, in the case of such a servo system, the servo loop is opened during the so-called jump operation for jumping recording track(s), and a jump pulse(s) is are applied to the actuator while the servo loop is opened, and afterwards the servo loop is closed at a predetermined timing so as to lead in the servo loop.
On the other hand, in the tracking servo system, an equalizer capable of performing a proportional-integral and differential (PID) operation, is generally provided in the servo loop, so that the servo system is stabilized by compensating the frequency characteristic of the tracking error signal by means of this equalizer. More specifically, during the normal playing operation a gain boost compensation is performed for the low frequency range of the total open loop characteristic by means of this equalizer. With this feature, the followability of the pickup is improved against the disc eccentricity component.
Since there is a correlation between the gain characteristic and the phase characteristic in the total open-loop characteristic of this equalizer, the phase characteristic becomes such that there is a phase inversion in the middle frequency range (about 1 KHz) as a result of a gain boost compensation effected in the low frequency range.
Now, the lock-in process of the servo loop after a jump operation will be described. Since the frequency of the jump operation is 1 through 2 KHz, a good phase characteristic around 1 KHz is required with respect to the servo characteristic. However, there is an inversion of the phase characteristic in the middle range as mentioned above, and the lock-in of the servo loop occurs while an acceleration action is working on the actuator. As a result, the overshoot becomes large, so that the lock-in process of the servo loop becomes unstable.